The lightweighting potential of composite materials is undeniable. But harnessing that potential in a reasonably economic way is the crux to widespread composites use in vehicle structures.
Within the Hyundai Motor Group conglomerate is a steel company, Hyundai Steel, so material challengers have to impress on several fronts—not just proving lighter weight—to overtake steel for a given vehicle application. Chi-Hoon Choi of Hyundai Motor Co., who holds a Ph.D. in Polymer Science and Engineering, said as much during the “Structural Plastic Composite Components: Pathway to Adoption” technical session at the SAE 2014 World Congress.
“High-strength steel (HSS) is one of the most useful materials when we want to reduce the weight of a body-in-white (BIW),” Choi said. Hyundai now employs HSS of 1.0 to 1.5 GPa using a hot stamping process. “So we have to develop a specific composite to replace that kind of part…If we want to apply composite materials instead of light metal or high-strength steel, we have to develop a unique design concept.”
One example was a battery pack case that Hyundai researchers developed for the i10 EV (electric vehicle). The goal was to develop a one-piece composite case as opposed to the steel case that consisted of more than 20 parts.
“The cost of the [initial CFRP] composite case was much higher than the steel one, so we decided to develop a PP [polypropylene] based product reinforced with glass fiber instead of CFRP [carbon-fiber-reinforced polymer],” Choi said. The result was a three-piece battery pack case that was 10 kg (22 lb) lighter—about 24 kg (53 lb) vs. 34 kg (75 lb) for the steel case.
Another current Hyundai project is the development of an exterior body panel—a hood, in particular—made of composite material. Phase I involves materials development and processing including injection molding and the RTM (resin transfer molding) process. Phase II—concept design and prototyping—will begin second half of 2014, according to Choi.
(Hyundai also revealed the Intrado concept vehicle at the 2014 Geneva Motor Show, which boasts a CFRP structure. Read more at http://articles.sae.org/12912/.)
The first phase of a technology roadmap Choi presented revolves around the execution of long, glass, semi-structural composites in the 2015 time frame. The second phase, in the 2020 time frame, consists of continuous, carbon, structural composites.
“Composites applications for structural parts will be the biggest challenge of lightweight technology,” he said. “We have to develop low-cost carbon fiber—around $10/kg—if it is possible.”
That price point—if not even less expensive—must be achieved before Ford seriously considers placing a significant amount of CFRP composites in its mainstream vehicles, as well.
“Certainly $30 or $40 a kilo—where carbon fiber is now—we’re not ready to put it on our Ford Focus, at 1.4 million vehicles a year, yet,” said David Wagner, Technical Leader in Vehicle Design, Research and Advanced Engineering at Ford Motor Co.
For the Multi-Material Lightweight Vehicle (MMLV) project, a collaboration between the U.S. Department of Energy, Vehma International Engineering (the U.S. R&D arm of Magna International), and Ford, the goals were to create a driveable prototype at a 25% weight reduction (Mach I) and to design a vehicle with a 50% weight reduction (Mach II). Unlike the Mach II design, which is carbon-fiber-intensive particularly in the body and chassis structures, the Mach I prototype makes more sparing use of the costly material.
“As far as the value to Ford Motor Co., it’s going to be awhile before we have a lot of carbon fiber in a Focus,” Wagner said. “So we’re prototyping the parts that are going to be most valuable to us in the near term.”
Those parts include a carbon-fiber instrument panel and cross-car beam that integrates the HVAC ducts, carbon-fiber seats, and carbon-fiber wheels from Carbon Revolution that are 45% lighter. In the powertrain area, Ford collaborated with BASF and others to use carbon fiber for the front cover, oil pan, and cam carrier (aluminum is still used for the head).
Though not of carbon fiber, glass-epoxy composite front coil springs from Sogefi are also featured on the MMLV and offer more than a 50% weight savings. Sogefi uses a low-volume “lost core” process to make the springs; the supplier is working to make it a high-volume production process, according to Wagner.
“This [process] is kind of equivalent to what our engine-block folks do all the time, so it’s not out of the question, it’s just different; it’s not how the steel guys make springs,” he said.
The prototype vehicle uses advanced high-strength steels and aluminum sheet, castings, and extrusions extensively for the BIW and closures, with a small magnesium casting employed in the front doors.
“We’re looking at [carbon fiber to be] at $5/lb; I wouldn’t buy it at $8 yet, and that’s the fiber only,” he said.
For the Mach II CAE design, most of the passenger cell consists of carbon-fiber composites. “The layups are spectacular,” Wagner said. “We don’t think this is quite manufacturable yet. The front rails are still likely to be in aluminum, and portions of the B-pillar will still be in boron steel because roof strength is really tough to achieve.”
At the midpoint in the design process, the Mach II BIW currently achieves a 45% weight reduction against the benchmark. The team would like to raise that figure to above 50%.
“For the longer term, you need to get [carbon fiber] into the body and chassis—saving tens of kilos rather than single digits of kilos. That’s going to be a long time,” said Wagner. “Aluminum vehicles are coming more strongly into the industry, and it’s going to be another decade before we see much headway for lots of carbon fiber in the body and chassis.”
Cost is the “overwhelming aspect” to this implementation delay, he added: “It’s cost of material, it’s cost of making the parts, it’s cost of assembling the parts, it’s cost of what changes you make to your vehicle assembly, and what changes you make in your coating systems…”